Part of the context is of course Iran also had it's democratically elected leader assassinated by the CIA in the 50's. Trust.
The US supports democracy when our allies win or we can rig the results, but not much when Hamas or the Islamic Salvation Front in Algeria or when a nationalist/anti-colonialist won in Iran). And after Iranians finally overthrew the Shah's dictatorship -- was he less autocratic or brutal than Saddam? -- we escorted the puppet we installed to safety and luxury instead of facing justice from his angry compatriots.
More recently, the US invaded the two countries on either side of Iran, killing and displacing large numbers, and destabilizing these lands leaving each with a capital city under siege barely controlling any of the mostly lawless country. That's what we've done to Iran's neighborhood. The US has been an out of control, warmongering nation, flouting international law for far too long. The great poet Louis Simpson once wrote "and is there no end to the wars of democracy." Apparently not, since he was writing during the Vietnam War ...
Why should Iran think the US is trustworthy or wouldn't bomb the hell out of Iran given half the chance. They saw up close how we decimated their neighbors.
Besides assassinating a military leader of Iran who was invited to Iraq, the US also obliterated/assassinated one of the heads of Iraq's own popular mobilization forces, a branch of the Iraqi military.
You might not like Assad, and the US went in to Syria to defeat the brutal faux caliphate of ISIS, but Iran's military was invited in to Syria by its lawful gov't to end a rebellion. The US already left Iraq once and was invited back in to fight ISIS and train the Iraqi military. Assassinating Iraqi and Iranian military leaders obviously wasn't part of any agreement and showed a total disregard for Iraq's sovereignty. The US complains that Iran tries to coopt and control Iran, but only because the US is in competition to own them.